1. Field of the Invention
The present invention relates generally to torsion springs and, more particularly, to a nonlinear torsion spring for use as a spring in a machine or as a stabilizer bar in a vehicle.
2. Description of the Prior Art
In designing vehicles such as automobiles and the like, it is known to use stabilizer bars to control the amount of body roll experienced during cornering. Typically, a passenger vehicle has two stabilizer bars, one working in conjunction with the front suspension, and the other with the rear. Each of these bars is conventionally mounted in two rubber bushings attached to the vehicle frame and is rotatable freely inside the frame bushings. Two arms extend outwardly from the central torsion element of the stabilizer bar and attach to suspension members at either side of the vehicle.
By this construction, when the left and right suspensions rise or fall together, the stabilizer bar merely rotates in its bushings and no force is imparted to either of the suspension members of the vehicle or to the vehicle frame. However, if the vehicle enters a turn, one side of the suspension rises and the other falls due to the roll of the vehicle body caused by the centrifugal forces acting thereon during the turn. This rolling action causes the torsion bars to twist and impart a force to the suspension members and vehicle frame which resists the vehicle's tendency to roll.
In designing stabilizer bars to provide the proper amount of resistance to the rolling forces of the vehicle body, it is conventional to vary the diameter of the stabilizer bar. The thicker the stabilizer bar is, the less roll the vehicle will be permitted to experience. However, the stabilizer bar also has an influence on straight line vehicle behavior which complicates the stabilizer bar design. When a vehicle is traveling in a straight path and one wheel encounters a hole or bump, the stabilizer bar imparts a force to the suspension member because the opposite suspension has not moved relative to the vehicle's frame. If the stabilizer bar is exceptionally stiff, the suspension will not be able to respond quickly enough to follow the contours of the road and the vehicle will roll when a rolling action is undesirable. Such a rolling action produces a feeling of instability for the driver and causes discomfort for passengers. On the other hand, if the stabilizer bar is made soft, the vehicle will experience excessive body roll during cornering. Neither of these situations is desirable.
One type of apparatus which has been designed to balance the effects of the stabilizer bar on the vehicle during straight line and cornering movement is exemplified in U.S. Pat. No. 4,621,831. In the device disclosed in this patent, a stabilizer bar is provided which may be turned on and off to permit the bar to be set either to provide a soft ride over bumps during straight line movement while causing no resistance to rolling during cornering, or to permit responsive handling during cornering while giving a rougher straight line ride. One obvious effect of such a device is that the stabilizer bar must either be completely on or completely off, thus producing sudden unpredictable changes in the roll behavior of the vehicle when going from one operational condition to the other.
An extension of the on-off concept is shown in U.S. Pat. No. 4,648,620, wherein an apparatus is provided having means for permitting a stabilizer bar to exhibit either of two possible spring rate settings. This construction permits the stabilizer bar to produce a soft rate providing a smooth ride in straight line travel of the vehicle and, upon activation of a selection solenoid, produces a stiff rate providing reduced roll during cornering. As with the above-mentioned on-off design, in the design employed in U.S. Pat. No. 4,648,620 a transition occurs when the stabilizer bar is switched from the first setting to the second setting and an abrupt change in the handling of the vehicle may be sensed by the operator. In addition, it is necessary that the vehicle be in a neutral position when the selection is made so that the operator must choose whether soft or hard characteristics are desired prior to putting the vehicle in gear and cannot easily switch between the two during movement of the vehicle.
Several problems are known to exist in conventional stabilizer bar devices. For example, because only one or two different linear spring rates are employed in any one stabilizer bar design of known type, the range of roll characteristics is limited and a choice as to the relative importance of straight line travel comfort versus cornering ability of the vehicle is unavoidable.